1. Field of the Invention
The present invention relates to the field of electro-acoustic transducers. More particularly the present invention relates to the field of reducing the distortion of electrodynamic loudspeakers.
2. Description of the prior Art
Electrodynamic loudspeakers are electro-acoustic transducers which transform varying electrical signals to corresponding audible signals. A conventional loudspeaker typically has a housing, a magnet, a diaphragm, a coil support connected longitudinally to the diaphragm, and a voice coil wound transversely on the coil support within the magnetic field of the magnet. When varying electrical signals are provided to the voice coil, a longitudinal force is exerted upon the voice coil. The polarity of the magnetic field, the winding direction of the voice coil, and the direction of the electric current through the voice coil determine the direction of the longitudinal movement of the voice coil. As the voice coil moves, the diaphragm moves sympathetically to develop the corresponding audible signals.
Over the years there have been numerous efforts to reduce the distortion of an electrodynamic loudspeaker. The following fourteen (14) Patents have been uncovered as being most pertinent prior art references related to the present invention:
1. U.S. Pat. No. 4,783,824 issued to Kobayashi on Nov. 8, 1988 for "Speaker Unit Having Two Voice Coils Wound Around A Common Coil Bobbin," hereafter "Kobayashi ('824)".
2. U.S. Pat. No. 4,609,784 issued to Miller on Sep. 2, 1986 for "Loudspeaker With Motional Feedback," hereafter "Miller ('784)".
3. U.S. Pat. No. 4,598,178 issued to Rollins on Jul. 1, 1986 for "Means For Critically Damping A Dynamic Loudspeaker," hereafter "Rollins ('178)".
4. U.S. Pat. No. 4,492,827 issued to Shintaku on Jan. 8, 1985 for "Horn Speaker With Reduced Magnetic Flux Leakage," hereafter "Shintaku ('827)".
5. U.S. Pat. No. 4,243,839 issued to Takahashi et. al on Jan. 6, 1981 for "Transducer With Flux Sensing Coils," hereafter "Takahashi ('839)".
6. U.S. Pat. No. 4,220,832 issued to Nagel on Sept. 2, 1980 for "Two-Way Speaker With Transformer-Coupled Split Coil," hereafter "Nagel ('832)".
7. U.S. Pat. No. 4,151,379 issued to Ashworth on Apr. 24, 1979 for "Electromagnetic Speaker With Bucking Parallel High And Low Frequency Coils Drives Via Magnetic Coupling And Having Adjustable Air Gap And Slot Pole Piece," hereafter "Ashworth ('379)".
8. U.S. Pat. No. 3,937,905 issued to Manger on Feb. 10, 1976 for "Moving Voice Coil Transducer Having A Flat Diaphragm Of An Impregnated Knit,"hereafter "Manger ('905)".
9. U.S. Pat. No. 3,686,446 issued to Manger on Aug. 22, 1972 for "Push-Pull Moving Coil Loudspeaker Having Electromagnetic Centering Means," hereafter "Manger ('446)".
10. U.S. Pat. No. 3,196,211 issued to Kessenich on Jul. 20, 1965 for "Speaker Arrangement," hereafter "Kessenich ('211)".
11. U.S. Pat. No. 3,067,366 issued to Hofman on Dec. 4, 1962 for "Magnet System Having Little Stray," hereafter "Hofman ('366)".
12. U.S. Pat. No. 3,047,661 issued to Winker on Jul. 31, 1962 for "High Fidelity Audio System," hereafter "Winker ('661)".
13. U.S. Pat. No. 2,007,749 issued to Anderson on Jul. 9, 1935 for "Acoustic Apparatus," hereafter "Anderson ('749)".
14. U.S. Pat. No. 2,007,748 issued to Olson on Jul. 9, 1935 for "Acoustic Device," hereafter "Olson ('748)".
Olson ('748) discloses an electrodynamic loudspeaker comprising a large voice coil 6 and a small voice coil 7 wound on a coil support 5 in two spaced apart locations to reduce distortion. Small voice coil 7 is proximal to a diaphragm 1 and large voice coil 6 is distal to diaphragm 1. In the electrical circuit shown in FIG. 2 of the Olson Patent, small voice coil 7 is connected in series with a capacitor 11, then the two are connected in parallel with large voice coil 6.
Anderson ('749) is issued on the same day as Olson ('748) and also discloses an electrodynamic loudspeaker comprising a small voice coil 30 located near a diaphragm 12 and a large voice coil 28 located remote from diaphragm 12. The difference in Anderson ('749) compared to Olson ('748), is that in the electrical circuit shown in FIG. 4 of Anderson ('749), large voice coil 28 is connected in parallel with a capacitor 40, then the two are connected in series with small voice coil 30.
Winker ('661) discloses an audio system comprising an electrodynamic loudspeaker. Referring to FIG. 1 of Winker ('661), an audio input signal 10 is fed to amplifier 11 where it is amplified and then fed to a voice coil 12 connected to a diaphragm 14. Voice coil 12 is located within a permanent magnetic field 15, so that the amplified signal passing through it will cause it to drive diaphragm 14 which in turn produces an audible signal. The distortion that occurs in the audio system is based on various factors including the inertia of loudspeaker 13 and of the audio system and any spurious oscillation developed in amplifier 11. In order to eliminate the distortion, a negative feedback which is a function of the speed and the displacement of diaphragm 14 is introduced into the audio system. The negative feedback system of Winker ('661) comprises two sensing units: a speed sensing unit and a displacement sensing unit. The speed sensing unit includes a sensor coil 28 wound on the same coil support as voice coil 12, and located within the same magnetic field 15. As diaphragm 14 moves, sensor 28 moves together and induces an electric current which is a function of the moving speed of diaphragm 14. The induced current is fed to a speed feedback network 29 which includes an invertor for signal phase inversion. The displacement sensing unit includes a mechanical device 17 mounted to diaphragm 14 and connected to a transducer 18 having its own power source 34. Transducer 18 transduces the relative displacement of mechanical device 17 to electric voltage which is a function of the moving displacement of diaphragm 14. The transduced voltage is fed to a demodulator 37 and then to a displacement feedback network 19 which also includes an invertor for signal phase inversion. The inverted signals from speed feedback network 29 and displacement feedback network 19 are then fed together to a preliminary summing network 30 to be combined to a negative feedback signal having the same amplitude but opposite polarity as the distortion. Finally, the negative feedback is fed into a final summing network 31 where the input audio signal is combined with a negative feedback signal having the same amplitude but opposite polarity as the distortion, and the combined input audio signal is then fed to amplifier 11 to eliminate the distortion of diaphragm 14.
Hofman ('366) discloses an electrodynamic loudspeaker comprising a magnet system with additional magnets to reduce stray. The magnet system includes a hollow permanent magnet 3, a magnetic core 1 concentric inside permanent magnet 3 with space in between, a top magnetic plate 4 having an aperture concentric With magnetic core 1, and a bottom magnetic plate 2 covering the opening in permanent magnet 3.
Kessenich ('211) discloses an electrodynamic loudspeaker comprising two voice coils 20 and 21 wound on a coil support 14 such that one is wound over the other. Voice coils 20 and 21 are wound in opposite directions and the electric currents provided to voice coils 20 and 21 are also in opposite directions. Therefore, the force in the longitudinal direction is enhanced and the other misalignment force in the transverse direction is balanced out.
Manger ('446) discloses an electrodynamic loudspeaker comprising an electromagnetic centering means having two driving coils 5 and 6 provided with electric currents for providing balanced forces to return a diaphragm 7 to a datum position so that the distortion at low frequencies can be reduced.
Manger ('905) discloses an electrodynamic loudspeaker having a similar arrangement as the first Manger Patent. The improvement is made to the diaphragm. The improved diaphragm comprises a flat textile carrier 14 impregnated with a highly attenuating filling material and is highly elastic in its plane but inelastic in bending, so that the distorted sound waves reproduced by the diaphragm can be reduced.
Ashworth ('379) discloses an electrodynamic loudspeaker comprising a treble voice coil 6 and a bass voice coil 7 wound on a single magnetizable coil supporting core 4 to activate a magnetizable sounding board 10 at the proximal end of coil supporting core 4, and to supply magnetic energy at the distal end of coil supporting core 4 for a second diaphragm 17 via another coil 15.
Nagel ('832) discloses a two-way electrodynamic loudspeaker system comprising a large loudspeaker 11 and a small loudspeaker 111. Large loudspeaker 11 has two voice coils 14a and 14b coupled by an input transformer 48 for increasing the energy efficiency over a broad frequency range.
Takahashi ('839) discloses an electrodynamic loudspeaker comprising two magnetic flux sensor coils for negative feedback. In the preferred embodiment of Takahashi ('839) shown in FIG. 1, a first sensor coil 6 is proximal to the diaphragm on one side of a voice coil 5, and a second sensor coil 7 is distal to the diaphragm on the other side of voice coil 5. When the feedback signals from sensor coils 6 and 7 are fed to an amplifier 8, a voltage in proportion to the change of magnetic flux is produced by provides a negative feedback force to the diaphragm that is proportional to the change of signal current flowing through voice coil 5. In an alternative embodiment of Takahashi ('839) shown in FIG. 16, a first sensor coil 27 is proximal to the diaphragm on one side of a voice coil 26, and a second sensor coil 28 is distal to the diaphragm on the other side of voice coil 26. When the feedback signals from sensor coils 27 and 28 are fed to an amplifier 29, a voltage in proportion to the change of magnetic flux is produced by amplifier 29. When an input signal current is flowing through voice coil 26 to drive the diaphragm, a voltage in proportion to the input signal current is presented across a resistor 30. The voltage from amplifier 29 and the voltage across resistor 30 are fed to a multiplier 31. As a result, multiplier 31 produces a feedback signal in proportion to the product of the change of the magnetic flux and the input signal current, which is theoretically proportional to the distortion of the current in voice coil 26. Therefore, when the feedback signal is fed to the negative terminal of the amplifier 32 and the audio input signal is fed to the positive terminal of the same amplifier 32, the combined input signal can eliminate the distortion in voice coil 26.
Shintaku ('827) discloses an electrodynamic loudspeaker comprising a secondary magnetic piece 17 in addition to a primary magnetic piece 16 to reduce the magnetic flux leakage.
Rollins ('178) discloses an electrodynamic loudspeaker with means for critically damping the movement of the diaphragm. In addition to a voice coil 24 wound on a coil support 23 and located within the magnetic field of a permanent magnet 28, Rollins ('827) comprises a driving coil 30 wound on the same coil support 23 at a location distal to the diaphragm such that it only enters the magnetic field of magnet 28 when the diaphragm moves at its extreme excursion. Upon the occurrence of such excursion, driving coil 30 can be selectively enabled by an external circuit to damp the movement of the diaphragm.
Miller ('784) discloses an electrodynamic loudspeaker comprising a voice coil 20 wound on a coupling 22 connected to a diaphragm 24 and located within a primary magnetic field, and two sensor coils 38 and 40 wound on the same coupling 22 and located within a second magnetic field. The two sensor coils are electrically connected in anti-phase.
Kobayashi ('824) discloses an electrodynamic loudspeaker comprising a first voice coil 24 located near the magnetic field of a first magnet 4 and a second voice coil 25 located near the magnetic field of a second magnet 5 which is provided for suppressing flux leakage of first magnet 4.
Overall, the prior art Patents can be approximately divided into three groups providing three different approaches to reduce the distortion of an electrodynamic loudspeaker. The approach of the first group of prior art patents is primarily to add more voice coils or other driving coils to balance the diaphragms of the loudspeakers. The first group includes Olson ('748), Anderson ('749), Kessenich ('211), Manger ('446) & ('905), Ashworth ('379), Nagel ('832), Rollins ('178) and Kobayashi ('824). The approach of the second group of prior art patents is primarily to add more magnets to balance the magnetic fields in the loudspeakers. The second group includes Hofman ('366) and Shintaku ('827). It should be pointed out that some of the prior art patents of the first group also have magnetic assemblies with extra magnets to balance the magnetic fields of the loudspeakers. The approach of the third group of the prior art patents is primarily to add one or more sensor coils to feedback signals which are functions of the motion of the diaphragms of the loudspeakers. The third group includes Winker ('661), Takahashi ('839) and Miller ('784).
The approach of the third group of the prior art patents has proven to be very effective in reducing the distortion of a loudspeaker. The main concept of this approach is to add sensor coils which move accordingly with the diaphragm of a loudspeaker for producing feedback signals. Winker ('661) uses a sensor coil 28 to detect the moving speed of the diaphragm. Since an accurate negative feedback signal should be a function of both the speed and the displacement of the diaphragm, Winker ('661) uses additional apparatus, such as a capacitor transducer, an ionization transducer or a pressure responsive resistor, to detect the displacement of the diaphragm. The major disadvantage of Winker ('661) is that its displacement detecting apparatus all comprise mechanical attachments, such as an arm 17, attached to the diaphragm, and the detecting components with their own power sources, such as multi-plates capacitors or pressure responsive resistors, have to be located adjacent to the mechanical attachments, which makes the loudspeaker somewhat too large and heavy thus not applicable for many compact devices such as earphones. In fact, for conventional markets there has been no practical loudspeaker designed and built such as the Winker ('661) loudspeaker. Takahashi ('839) later provides a different solution by using two sensor coils on each side of the voice coil to measure the change of the magnetic flux of the magnetic field when the diaphragm is moving and feeding back the feedback signal to a driving coil which in turn provides a negative feedback force to the diaphragm that is proportional to the change of signal current flowing through the voice coil. The Takahashi ('839) loudspeaker also requires a larger housing for the loudspeaker because of the numerous extra coils and complicated magnet assemblies. Miller ('784) is concerned about having the sensor coil located together with the voice coil within the same primary magnetic field, such as Winker ('661) does, because of the electromagnetic interference, the so-called "transformer effect," between them. Miller ('784) provides a simple assembly having a pair of sensor coils electrically connected in anti-phase and separately located in a secondary magnetic field away from the voice coil and the primary magnetic field. However, it is known that such a sensor coil arrangement is merely adequate for detecting the moving speed of the diaphragm, as discussed in Takahashi ('839) from Line 4, Column 9 to Line 32, Column 10, and that is why Winker ('661) employs additional apparatus to detect the displacement of the diaphragm.
Referring to FIG. 1, there is shown the circuitry and arrangement of a most relevant prior art conventional electrodynamic loudspeaker 100. An audio input signal 10 is input to an amplifier 20 where it is amplified and sent to loudspeaker 100. Loudspeaker 100 comprises a primary magnet 110, a secondary magnet 120, a diaphragm 130, a coil support 140 attached to diaphragm 130, a voice coil 150 wound on coil support 140 and located within the magnetic field of primary magnet 110 which is referred to as the primary magnetic field, and a sensor coil 160 wound on coil support 140 and located within the magnetic field of secondary magnet 120 which is referred to as the secondary magnetic field. The amplified input signal is sent into and passes through voice coil 150 and causes voice coil 150 to drive diaphragm 130 to move back and forth because of the presence of the primary magnetic field. As diaphragm 130 moves back and forth, so does sensor coil 160 which transduces a first feedback signal that is proportional to the moving speed of diaphragm 130. This first feedback signal is fed to a speed feedback network 30 which includes an invertor for signal phase inversion. As diaphragm 130 moves back and forth, a mechanical attachment 42 attached to diaphragm 130 also moves accordingly and a transducer 40 transduces the displacement of diaphragm 130 to an electronic signal. Transducer 40 is energized by its own power source 50. The electronic signal from transducer 40 is fed to a modulator 60, which in turn produces a second feedback signal that is proportional to the displacement of diaphragm 130. This second feedback signal is fed to a displacement feedback network 70 which also includes an invertor for signal phase inversion. Speed feedback network 30 then produces a first negative feedback signal at 180 degrees out of phase with the first feedback signal and feeds the first negative feedback signal in the proper proportion into a preliminary summing network 80. Displacement feedback network 70 also produces a second negative feedback signal at 180 degrees out of phase with the second feedback signal and feeds the second negative feedback signal in the proper proportion into preliminary summing network 80. The preliminary summing network 80 combines the first and second negative feedback signals and produces a combined negative feedback signal which is a function of both the speed and the displacement of diaphragm 130. The combined negative feedback signal is fed in the proper proportion to a final summing network 90 where it is combined with the input audio signal Finally the combined input signal is sent into loudspeaker 100 and the distortion of diaphragm 130 is thus eliminated.